Catalytic processes for the conversion of hydrocarbons are well known and extensively used. Invariably the catalysts used in these processes become deactivated for one or more reasons. Where the accumulation of coke deposits causes the deactivation, reconditioning of the catalyst to remove coke deposits helps restore the activity of the catalyst. Coke is normally removed from catalyst by contact of the coke-containing catalyst at high temperature with an oxygen-containing gas to combust and remove the coke in a regeneration process. These processes can be carried out in-situ or the catalyst may be removed from a vessel in which the hydrocarbon conversion takes place and transported to a separate regeneration zone for coke removal. Arrangements for continuously or semi-continuously removing catalyst particles from a reaction zone and for coke removal in a regeneration zone are well known.
In order to transport catalyst from a reaction zone containing hydrocarbons to a regeneration zone containing oxygen, the two zones are ordinarily connected by a pipeline. The two zones generally operate at different pressures and contain different fluids which preferably are inhibited from communicating with one another. Communication between the two zones through the connecting pipeline can be inhibited by closing valves in the line, but this also stops the flow of catalyst between the zones. Besides, valves that close in pipelines containing flowing catalyst are generally maintenance problems, because of leakage due to wear from catalyst particles and malfunction due to high temperatures.
Communication between the two zones can also be inhibited by introducing an inert fluid into the connecting pipeline while catalyst flows between the zones. The inert fluid is generally introduced into a section of the pipeline at a pressure higher than the pressures of the two zones. Beginning at the point of introduction of the inert fluid, two portions of the inert fluid flow through the pipeline in opposite directions--one portion toward one zone and the other toward the other zone. Therefore, relative to the flow of the inert fluid, in one leg of the connecting pipeline catalyst flows countercurrently whereas in the other leg catalyst flows concurrently.
One drawback associated with the introduction of an inert fluid into the connecting pipeline between two zones is the length of the legs of the connecting pipeline. The length of the legs is determined by the dual design criteria of inhibiting communication and preventing the flow of inert fluid from neither overly hindering nor overly assisting the catalyst flow through the legs. As an illustration, for a vertically-disposed and moderately-sized hydrocarbon conversion reaction vessel with one connecting pipeline on its catalyst outlet, the height of the superstructure that supports the vessel and the connecting pipeline may be 50-100% taller than the superstructure of the vessel without a connecting pipeline. When a second connecting pipeline is also employed--e.g., on the catalyst inlet--the height and cost of the superstructure are compounded.